Lambda N negative cIts2 prophages will be inserted into bacterial chromosomes lacking the lambda attachment region. The bacteria will first be lysogenned with lambda N positive cIts857, and then superinfected with lambda N negative cIts2h. From lysogens containing both prophages, segregants containing only lambda N negative cIts2h will arise by recombination. If the lysogens are killed after derepression of the prophage, we will produce polylysogens by superinfection with a high m.o.i. of lambda N negative cIts2. Such polylysogens, with lambda at the normal attachment site, grow slowly after derepression, producing distinctive "converted colonies." To simplify the mapping of the site at which lambda has inserted, we will prepare Hfr derivatives of strain B583, which is deleted to att lambda and gal. F's containing lac will be transferred into the strain, and stable lac positive bacteria selected with acridine orange. Selection by growth in a minimal medium containing penicillin will be used to isolate lysogens in which lambda insertion results in auxotrophy. BIBLIOGRAPHY REFERENCES: Lieb, M. 1976. Mapping missense and nonsense mutations in gene cI of bacteriophage lambda: marker effects. Molec gen. Genetics 146:285-290. Lieb, M. 1976. cI mutants: Intragenic complementation and complementation with a cI promoter mutant. Molec. gen. Genetics 146:291-297.